1. Field of the Invention
The invention relates to a measurement transducer for process instrumentation having a sensor for detecting a physical or chemical quantity and to a method for monitoring the state of the sensor.
2. Description of the Related Art
Various field devices for the process instrumentation are used in process engineering systems to control processes. Measurement transducers are used, for instance, to detect process variables, such as temperature, pressure, through-flow rate, fill level, density or gas concentration of a medium. With the aid of a sensor, which is frequently attached to a planar substrate, the physical or chemical quantity is converted into an electrical measuring signal. This planar substrate is then used to mechanically fix the sensitive sensor to a support and also to protect against external influences, such as to improve electromagnetic compatibility. Particularly, with piezoresistive pressure sensors, there is the possibility of attaching electrical sensor elements of the sensor to the substrate in that these elements are embedded in the substrate and the substrate is doped in the vicinity of these elements, where electrical contact is established between the electrical element and the substrate. A PN junction in the manner of a diode provides for electrical separation of the element and the substrate when an adequate voltage is applied. Electric lines, which convey signals from the sensor to external contacts, are connected to an actuation and evaluation facility to evaluate the signals, where the facility outputs a measured value corresponding to the respective pressure to a control station or a programmable logic controller via a field bus.
EP 2 269 009 B1 discloses a measurement transducer for process instrumentation, in which the state of a substrate, which supports electrical elements of a sensor to generate a measuring signal, is monitored for changes due to chemical contamination. In addition, the sensor elements and their connecting lines are monitored for breakage, in other words for the occurrence of an electrical interruption.
The detection of a sensor breakage can occur, for instance, by measuring the current consumption of the measuring resistances connected to a Wheatstone bridge, where the measuring resistances generally are referred to as sensor elements. The temperature sensitivity of these resistances is relatively high compared with their pressure sensitivity. As a result, an extremely accurate determination of the consumed current with additional temperature compensation is required. The extremely precise measuring electronics required herefor is disadvantageously associated with a high outlay on account of the required low component distribution. Since more and more applications of measurement transducers require very high reliability when measuring physical or chemical quantities, which is to be confirmed by corresponding certifications, for instance pertaining to International Electrotechnical Commission (IEC) standard 61508, it is however rarely possible to dispense with monitoring the sensor for malfunctions or breakages. In such cases, a malfunction of a sensor can inter alia be caused by a tearing of signal and/or supply wires, which can be formed as lines on the substrate or as bonding wires, or for instance in the case of resistive pressure sensors by a breakage of the material doped on the substrate.